The present invention relates to a method for producing hard plaster of paris having very short setting times and high strength values.
Hard plaster of paris refers to calcium sulfate hemihydrates which contain as their major component alpha calcium sulfate hemihydrate. During the setting process, these hemihydrates rehydrate to form the dihydrate, and as indicated by the term "hard plaster of paris" they reach high strength values after setting and are therefore very well suited, according to U.S. Pat. No. 1,901,051, for the production of plaster casts. Due to the high strength values of the plaster, the weight of the plaster casts can be kept relatively low.
The plaster of paris used for plaster casts must have a short setting time, about three minutes or less being desirable, as well as a high degree of whiteness. Moreover, during setting the plaster casts should not secrete a film of water on their surfaces, and the setting process should not produce too high an exothermal reaction which could lead to skin burns.
In U.S. Pat. No. 3,913,571 it is proposed, for the production of plaster casts, to use an alpha calcium sulfate hemihydrate which has been produced by means of a hydrothermal recrystallization process and has been comminuted to a prescribed average grain size by way of very fine grinding in a grinding assembly. In accordance with the process disclosed in U.S. Pat. No. 3,913,571, the use of hydrothermally produced alpha calcium sulfate hemihydrate which has been ground to an average grain size of less than 7.mu., preferably less than 5.mu., results in improved strength characteristics and good processing properties. Optimum results are attained if the axial ratio of the hemihydrate crystals before grinding is at least 1:2 and no more than 1:5. One drawback of this process is, however, that the very fine grinding causes the development of excessive heat during setting.
Further, it is known that the setting times of alpha calcium sulfate hemihydrate are much longer than those of beta calcium sulfate hemihydrate. For that reason, accelerators such as calcium sulfate dihydrate and potassium sulfate are generally added to the hard plasters used in plaster casts. Both of these substances have an accelerating effect when dispersed very finely. The addition of beta calcium sulfate hemihydrate causes no or only an insufficient accelerating effect.
Mixtures of alpha and beta calcium hemihydrate are known and are used in industry in various fields. Due to their high water absorption capability they are used extensively, for example, in the ceramics industry, according to German Offenlegungsschrift No. 1,805,126, as a modelling and molding plaster of paris. However, the plaster mixtures preferred for these purposes, which consist of 15 to 40 percent by weight alpha hemihydrate and 85-60 percent by weight beta hemihydrate, do not exhibit the short setting times and increased mechanical strength which is required of plasters used in other applications, for example, for medical purposes. The addition of accelerators to these mixtures is necessary to attain reasonable setting periods, and the addition of these accelerators leads to a reduction in strength of the plaster.